JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
<li><label class="leftField fl">Room 1</label><div class="rightField txtbg"><select name="fromBox" class="selectArea required" id="sel">
  <option value="0">...Select...</option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
  <option value="5">5</option>
</select></div></li>
<li><label class="leftField fl">Room 2</label><div class="rightField txtbg"><select name="fromBox" class="selectArea required">
  <option value="0">...Select...</option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
  <option value="5">5</option>
</select></div></li>
<li><label class="leftField fl">Room 3</label><div class="rightField txtbg"><select name="fromBox" class="selectArea required">
  <option value="0">...Select...</option>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="3">3</option>
  <option value="4">4</option>
  <option value="5">5</option>
</select></div></li>
</ul>

JavaScript

$(function() {
    $('#sel').change(function() {
        var a = $(this).val();
        $("select[name=fromBox] option").each(function() {
            var b = $(this).val();
            if (a == b) {
            $(this).hide();
            }                
        });
    });